| In the researches of the formation mechanism of bio-oil from biomass pyrolysis,there are always blind spots in the study of heavy components,which hinders the development of bio-oil utilization technology.In this paper,cellulose,hemicellulos(xylan for model compound)and lignin were directly used as raw materials to study the formation and evolution of the heavy components of the pyrolysis bio-oil under different reaction temperatures and different reaction times by using fourier transform ion cyclotron resonance mass spectrometry,aiming to provide theoretical support for the formation mechanism of the heavy components and the high-value utilization of the pyrolysis oil.Firstly,the heavy oil gain from pyrolysis of biomass components at 400℃,500℃,600℃ and 700℃ of 90 s was detected and analyzed.The chemical properties,molecular weight distribution and species distribution of lignin heavy oil are different from those of other main components.The molecular mass of heavy oil of cellulose and xylan mainly concentrates in the range of 200-350 Da,while that of lignin mainly concentrates in the range of 300-400 Da.The main components of heavy oil in cellulose and xylan are phenolic-like species and sugars,while lignin mainly generates lipids and unsaturated hydrocarbons.With the increase of temperature,the heavy oils of the three components have the tendency of condensation.More lipids,unsaturated hydrocarbons and condensed aromatics with larger molecular weight were generated from further pyrolysis of saccharides and phenols in heavy oils during cellulose pyrolysis.The heavy components of lignin are more abundant than those of cellulose and hemicellulose.Lignin contains a large number of side chain with benzene ring structure which are tend to be cracked in the process of pyrolysis,forming a large number of lipids and unsaturated hydrocarbons.Higher temperatures promote the thickening of tar,resulting in more condensed aromatic compounds in the products than cellulose and hemicellulose.It is also found that with the increase of temperature,oxygen-containing functional groups play an important role in the formation and transformation of the heavy components during pyrolysis.Then,the pyrolysis experiments of the three components was carried out in 60 s at the temperatures of 500℃,600℃ and 700℃,and the results were compared with the previous experimental results to explore the evolution of heavy oil during pyrolysis.The results show that the abundance of heavy oil produced by cellulose and xylan in the initial pyrolysis stage is significantly greater than that in the secondary pyrolysis stage,and the difference between the abundance of heavy oil in the two stages decreases gradually with the increase of temperature,while the high molecular weight substances in lignin heavy oil increased significantly with the increase of residence time.In addition,the increase of temperature in the secondary reaction stage will promote the further reactions of phenols and sugars,and is conducive to the formation of condensed aromatic compounds.The pyrolysis process of xylan was accelerated by increasing temperature,while the secondary reaction of phenols was inhibited by high temperature.The secondary reactions between lipids and unsaturated hydrocarbons in lignin heavy oils are inhibited with increasing temperature,too.Kendrick mass defect method was used to study the reaction of heavy oil with the increase of residence time.Many of the heavy oil components of cellulose undergo complex pyrolysis reactions such as carbon chain cracking/addition,while the lipids and some unsaturated hydrocarbons produced in the secondary pyrolysis mainly undergo alkane chain growth.The secondary reaction of xylan heavy oil is mainly homologous evolution instead of the breaking of carbon chain.Due to the special structure of lignin,demethoxy and decarbonyl reactions mainly occur in the transformation of oxygen-containing substances during pyrolysis. |
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